The UN Security Council will vote Saturday on a Western-drafted resolution allowing a ceasefire observer mission in Syria even though Russia's support is in doubt.

The United States called for the vote after a second day of wrangling with Russia over security guarantees for the first 30 unarmed military monitors who UN-Arab League envoy Kofi Annan wants in Syria early next week.

Russia also opposed the council demanding that President Bashar al-Assad carry out a promise to withdraw troops and heavy weapons from Syrian cities.

Russia's UN ambassador Vitaly Churkin said he was not "completely satisfied" with the talks held at the UN on Friday. Russia and China have vetoed two previous Security Council resolutions on Syria.

Meanwhile, forces loyal to Assad shelled the Jurat al-Shayah and Al-Qarabis districts of the city of Homs for around an hour Saturday, the Syrian Observatory for Human Rights said, without giving any immediate word on any casualties.

Negotiations had been "rather difficult," he said while insisting that Russia wants a vote on Saturday that allows the Syrian ceasefire to be "reinforced."

Neither the United States nor its allies are certain that the resolution would escape a new veto.

"It would be wise not to make predictions," said US ambassador Susan Rice.

"There was a negotiation, there is not yet an agreement," France's UN Ambassador Gerard Araud told reporters. "It's very tough, but there will be a vote tomorrow in any case."

"A veto cannot be ruled out," said one Security Council diplomat. "It would be catastrophic for Syria if it happens."

A new version of the resolution drafted by the United States with Britain and France was sent to other council members late Friday for national governments to decide which way to vote.

Russia has also registered a shorter version of the draft for an eventual vote.

Both versions authorize the first 30 monitors in a force in Syria that would grow to more than 200 if the ceasefire started on Thursday holds. The UN says well over 9,000 people have been killed since an uprising against Assad started 13 months ago.

Annan has asked for approval for the monitors and for the council to call for all six points of his peace plan to be carried out. The Syrian government has yet to pull troops and heavy weapons from protest cities.

The text proposed by the United States, Britain, France, Germany, Morocco and Colombia demands "full, unimpeded, and immediate freedom of movement" for the observers and that Assad "implement visibly" commitments made to Annan.

The council could also "consider further steps as appropriate".

Russia submitted a shorter version of the same text taking out the demand for "unimpeded" access for monitors and the warning of new measures. It also takes out a condemnation of human rights abuses in Syria.

A diplomat in the negotiations said Russia, the last major ally of Assad, had been "haggling over every phrase" in the draft text.

Churkin said Russia wanted a brief resolution to get "some boots on the ground" and then negotiate the mandate for the full mission.

Despite their past vetoes on Syria, Russia and China have strongly committed to Annan's six-point peace plan and say they are putting increased pressure on Damascus.

The United States and European powers say however that there must be specific security guarantees and terms set out to the Syrian government before the advance mission leaves.

"This advance team should go with the same mandate as the full observation mission. We cannot accept that they go with weaker rules," said a Western diplomat in the talks.

The Security Council has passed three "presidential statements" but no formal resolution on Syria since the uprising against Assad started in mid-March last year.